1. Field of the Invention
The present invention relates to a spiral track-forming apparatus, for forming spiral-shaped recording tracks by feeding an optical recording system in a radial direction of a disk-shaped substance, and more particularly to a spiral track-forming apparatus which can form recording tracks of high density.
2. Description of the Prior Art
Generally, optical track-forming apparatus are known to use the method by which spiral-shaped recording tracks are formed in a photoresist coating on a disk-shaped substrate made from glass. These disks rotate at a fixed angular velocity, and an optical recording system moves in a straight line with and a fixed velocity in the radial direction on the substrate disk. In the spiral track-forming apparatus described above, in order to move the optical recording system in the radial direction, a positioning control system is used. With this positioning control system, the optical recording system is moved step-wise in accordance with a pulse signal from a length measuring instrument for detecting the position of the optical recording system. For this reason, the position of the recording track on the glass substrate disk is altered in a step-wise form corresponding to the step-wise movement of the optical recording system.
In recent years, there has been a need for optical recording in a high density mode on the glass substrate disk, and a means for making narrower the spacing between the recording tracks (track pitch) is usually considered. There, as a positioning control system, a system having a high resolution of positioning is required. In other words, it is necessary to have a high resolution device as a length measuring instrument for position detection for control of the positioning. However, there are limits to converting a length measuring instrument to high resolution to conform to the reduction (shrinking) of the pitch, and it is not practical. In addition, as described previously, in the case of using a positioning control system, the position of the recorded track does not change smoothly, but rather in a step-wise manner, therefore, if the track pitch is narrowed, adjacent tracks are easily caused to cross. This fact, in the case where the track pitch is narrowed and a high densification of the recording is produced, means that it is very difficult to control the radial feed of the optical recording system with the abovementioned conventional positioning control method. And if this system is executed, track pitch error is produced, and a true recording is not possible.